This download looks like an addon. You need to extract the downloaded zip and put the addon folder in your Garry's Mod addons folder. You should end up with a directory structure similar to below:
Don't know where the Garry's Mod folder is or how to extract zip files? See more useful installation tips.
benicioc1, No one expects the pancake, ThaMech202, andresdeleonpaniagua1997, rosethedemonfox, Lt. BrianBigBoss, Seeman, davideaffuso2001, youyou.bf, brajlovic.denis, brandonsouza95, alex.isickow, alex.isickow, dani_herrera_2002, silvershadow241, Deal with it, dmgeer01, Nightmare Balloon, aionitoaeiandrei, Vaporeon, Flareon and Jolteon, SCREAMOguy413, DutchyDD, Hunter, ♥MissMoo~♪, TheGameExplorer, Thunderblade, techbirdjr, juanluis19102003, mozzarellarella, midi, Pyroshi|Looking For HL2 Players, jacksackboy, rosimarrodriguessilva, The Empress, kirby, apjm17, thatguynamedbob, The Cold Hoovy, Tom Cat, XiaoWu0v0, API™, amstert, shadic20025, Boomergorock, Candy_the_cat2, Loltroll381, Raptor Hawk, nicowx, Simon 'Rai' Zander, nuct +231 others